The present invention relates to an apparatus for measuring a distortion factor of an analog input signal.
As is well known in the art a distortion factor K(%) is defined by the following equation; ##EQU1## wherein E.sub.1 is an effective value of a fundamental wave of the input signal, and Em is an effective value of an M.sup.th harmonic wave.
FIG. 1 is a circuit diagram showing a known apparatus for measuring a distortion factor. An input signal is supplied from an input terminal I to a switch S.sub.1 through a first variable attenuator 1 and a buffer amplifier 2. The input signal is directly supplied by means of a switch S.sub.2 to a second variable attenuator 4 and is supplied through a filter 3 which may be formed by, for example a twin T circuit or a Wien bridge for eliminating or suppressing a fundamental wave in the input signal to be measured. The signal is further supplied through an amplifier 5 to a level meter 6. The input signal is supplied from an oscillator through an amplifier or electronic circuit under test.
Upon measurement at first the switches S.sub.1 and S.sub.2 are connected to contacts 1 and the level of the input signal is measured. Then the switches S.sub.1 and S.sub.2 are shifted to contacts 2 and the fundamental component is removed from the input signal by means of the filter 3 and the level of harmonic waves is measured. Then the distortion factor can be obtained as a ratio between the two measured signal levels in accordance with the above mentioned equation.
Nowadays due to progress of semiconductor devices and circuit techniques properties of audio apparatuses have been improved to a great extent and these apparatuses have extremely high fidelity. Accordingly the apparatus for measuring distortion factors of such audio apparatuses should have a sufficiently high sensitivity for measuring a very small distortion factor such as 0.0003%. However, in order to measure such a low distortion factor by means of the known apparatuses, an operator would encounter the following difficulties.
(1) Since the fundamental wave should be removed from the input signal by the filter 3 to a great extent, it is quite difficult to construct the filter 3 which can be tuned to a desired resonance frequency with a given amount of attenuation. Usually the filter is composed of the twin T circuit or Wien bridge and is tuned to the desired frequency by adjusting a variable capacitor or resistor forming the bridge circuit. When an amount of component to be removed is increased, resonance characteristics should be very sharp and thus the amount of removed fundamental wave is limited to about 70 dB by means of manual adjustment. It is impossible to remove the fundamental component more than that, while the bridge circuit is kept in a balanced condition stably. If a plurality of filters are connected in cascade, an attenuation bandwidth will be widened and the tuning adjustment will be easy. However in such a case the capacitors and/or resistors of these filters have to be adjusted in conjunction with each other in a very accurate manner. Further the construction of the filter will be complicated and the cost will be increased.
(2) Noise which is produced in the distortion factor measuring apparatus should be negligible with respect to the distortion component in the input signal to be measured and could not be ignored. However, in the known apparatuses small distortion factors could not be measured with high accuracy due to the noise.